Device for the absorption of the expansion of a charge in a retort used in a nitriding process

ABSTRACT

A nitriding retort containing a means which forms pockets to absorb expansion of the nitrided material during the nitriding process.

during the nitriding process.

SIWWWWIS United States Patent [1113,589,695

, [72] Inventor Arne Ivar Backslrom [56] R fe Cit d I N gfg UNITEDSTATES PATENTS f 26 1968 799,696 9/1905 Ward 266/20X [45] Patented June'1971 3,116,842 1/1964 Mohr... 266/27 X Assignee Aves Jemverks Akfiebolag3,252,601 /1966 isaacs 266/27 X Avesm Sweden FOREIGN PATENTS [32]Priority Sept. 11, 1967 1,068,021 /1959 Germany 266/ [33] Sweden 779,8717/1957 Great Britain.. 266/20 [31] 12495/67 204,065 5/1966 Sweden 266/2Primary Examiner-J. Spencer Overholser [54] DEVICE FOR THE ABSORPTION OFTHE Assistant Examiner-R. Spencer Annear EXPANSION OF A CHARGE IN ARETORT USED IN Attorney-Brumbaugh, Graves, Donohue & Raymond A NITRIDINGPROCESS 8 Claims, 8 Drawing Figs.

[52] US. Cl 266/2 R [51] C23cll/l4 ABSTRACT: A nitriding retortcontaining a means which Field of Search 266/2, 2A, forms pockets toabsorb expansion of the nitrided material DEVICE FOR THE ABSORPTION OFTHE EXPANSION OF A CHARGE IN A RETORT USED IN A NITRIIDING PROCESSNitrogen is included as an alloying element in types of steel possessingcertain qualities, i.e. in stainless, austenitic steel. In steelproduction the nitrogen is added in a combined form as metal nitride, inwhich the metal may consists of manganese, ferromanganese, chromium orferrochromium.

Nitriding the metal is carried out in the solid phase, nor mally at 900to 1,000 C. The nitrogen supply source is generally nitrogen gas. Sincethe nitrogen diffuses into the metal extremely slowly, the latter mustpresent a large surface area if the time the treatment takesis not to beunreasonably long. The metal is therefore treated in the shape of flakesof a thickness of about I to 2 mm., for instance manganese metalproduced electrolytically or in the shape of a crushed product producedwith a grain size less than mm., for instance crushed ferrochromium,manganese or ferromanganese produced pyrometallurgically.

Suitable equipment for the nitriding operation is a retort ofheat-resistant steel which during the treatment is placed vertically inan electric pit furnace. The retort should preferably have a circularcross section, and it should be somewhat coneshaped, in order tofacilitate the discharging of the material.

In the nitration process the volume of the charge increases by about 5to percent, the charge will thus exert a considerable pressure on thewall of the retort. Since the strength of the retort material is ratherpoor at 900 to 1,000 C. the

result will imply that the retort is deformed and/or that cracks appearin it.

The deformation appears as a bulge which is often largest at about halfthe height of the retort and which may be so extensive that the retorthas to be discarded after a comparatively short time of operation. Therisk of cracking increases in proportion to the heating cyclesperformed, the welds are especially delicate areas.

The object of the present invention is to prevent the charge in thecourse of the volume increase in connection with the nitriding fromexerting such a pressure on the wall of the retort that the retort isdeformed or that cracks appear in it.

The present invention is characterized in that the retort is providedwith a number of discs preferably orientated centrally, horizontally andin a vertical pile. When the retort is charged vacant spaces developunder the discs. In the nitriding process these vacant spaces areentirely or partly filled by the expanding material. In this case thediscs may suitably be arranged at a distance from each other by means ofspacer members. According to a preferred embodiment of the presentinvention the spacer members consist of centrally arranged sleeves,preferably welded onto one disc each, the discs being provided withholes so that the discs and sleeves may be threaded on a centralremovable stand attached to the retort.

The present invention will be described in the following, referencebeing made to the attached drawings where FIG. 1 shows a vertical crosssection of a retort with an expansion absorber according to the presentinvention,

FIG. 2a schematically shows the above absorber during charging of theretort,

FIG. 2b schematically shows part of the retort after the charging.

FIG. 20 schematically shows part of the retort during heat treatment,

FIG. 2d schematically shows part of the retort after completed heattreatment,

FIG. 3 shows a disc with an attached spacer member, partly in section,

FIG. 4 shows a view from above of the disc in FIG. 3, and

FIG. 5 shows another embodiment of the disc, partly in section.

FIG. 1 shows retort l with a circular section. The retort has acircular-cylindrical body 2, a cone-shaped lower part 3 and a roundedbottom part 4. In operation the retort is covered by lid 5 which, viaflanges 6 on body 2, may be fastened to the re tort body.

An expansion absorber 7 has been placed centrally in the retort. Theabsorber consists of a vertical metal stand 8 provided with a number ofrounded metal discs 9 horizontally placed and with their centerscoinciding with the vertical stand. A device should be provided with oneor several supporting devices which maintain expansion absorber 7 in itsvertical, central position. At the top the stand ends in a loop forhoisting expansion absorber 7 and the nitrated cake after the finalreaction. For the supply of nitrogen gas there are holes (not shown) inthe bottom.

The way in which metal material 10 which is to be nitrided is fed intothe retort is illustrated in FIG. 2a. The material flows into the retortdown vertically, then it is symmetrically distributed in the retort.Under each circular disc 9 vacant space 11 is formed in the shape of aninverted cone or a frustum of a cone whose top rake angle is minus twicethe angle of repose a of the metal material, FIG. 2b.

When the retort is heated to reaction temperature the metal starts toreact with the nitrogen atmosphere forming metal nitride or nitrides. Inthe process of expansion of the material which is then starting, thematerial is pressed or expands radially in a direction towards thecenter shaft of the retort. The vacant space 11 under discs 9 are thenreduced according to FIG. 2c.

After completed reaction and expansion vacant spaces 11 under discs 9have been practically filled by the expanding material according to FIG.d, provided that the total volume of the vacant spaces corresponds to orsomewhat exceeds the expansion of the charge expressed in volume units.

FIGS. 3, 4 and 5 show examples of the way in which a disc and a spacermember may be designed in practice. According to FIGS. 3 and 4, circulardisc 12 is plane but provided with edge bevelling on the upper surface.Onto the lower surface of the disc guide sleeve 14 has been welded,which serves as a spacer member for keeping the desired distance to theunderlying disc not shown, and three stiffening ribs IS. The expansionabsorber consists of a number of such disc threaded onto central stand16, which may, for instance, consist of a pipe or a round bar. The wallof the retort is marked 17.

FIG. 5 shows a construction where disc I8 has been shaped into anobtuse-angled cone with a top rake angle chosen so that an assumed linedrawn from central stand 19 and in the plane of the disc towards wall 20of the retort forms a right angle to the latter in the vertical plane.As to the rest, this embodiment corresponds with the one shown in FIGS.3 and 4.

The present invention is not restricted exclusively to the constructiveembodiments described in greater detailabove.

For instance, the discs need not necessarily be of a circular shape.Square, pentagon or hexagon discs, etc. may very well be used even ifstereometrically the circle is the logical shape.

I claim:

1. A retort for nitriding metal particles at elevated temperaturescomprising a vertically elongate container for retaining the metalparticles therein in a substantially stationary mass during nitriding,means for reducing forces to the container by accommodating expansion ofthe mass of metal particles therein, the force reducing means includinga plurality of means for forming pockets within the mass of particles topermit inward expansion of the mass, the pocket forming means includinga plurality of members spaced-apart, positioned to interrupt the flow ofparticles into the container during filling to produce said pocketsbeneath each of the members, and supported throughout the fill depth toform an axially extending series of pockets throughout the particulatemass.

2. The retort according to claim ll, wherein the spaced members are aplurality of discs supported on a shaft, the shaft extends axially andsupports the disc throughout the fill depth of the retort, and the shaftand discs are removable from within the retort following nitriding tofacilitate removal of the nitrided mass of particles.

comprise sleeves attached perpendicularly and centrally between thediscs such that a through hole is formed for the insertion of asupporting stand which may be attached to the retort.

8. A retort according to claim 7 wherein the discs and sleeves may bepositioned on the stand by means of threads cut into the exteriorsurface of the stand and corresponding threads cut into the interiorsurface of the sleeves and the interior surface of the holes provided inthe discs.

2. The retort according to claim 1, wherein the spaced members are aplurality of discs supported on a shaft, the shaft extends axially andsupports the disc throughout the fill depth of the retort, and the shaftand discs are removable from within the retort following nitriding tofacilitate removal of the nitrided mass of particles.
 3. A retortaccording to claim 2, wherein said discs are essentially circular inshape.
 4. A retort according to claim 2 wherein said container has anessentially circular horizontal cross section.
 5. A retort according toclaim 4 wherein said container is essentially cone-shaped with a roundedbase.
 6. A retort according to claim 2 wherein the horizontal discs areseparated from each other by means of spacer members.
 7. A retortaccording to claim 6 wherein the discs are provided with a centrallylocated hole and said spacer members comprise sleeves attachedperpendicularly and centrally between the discs such that a through holeis formed for the insertion of a supporting stand which may be attachedto the retort.
 8. A retort according to claim 7 wherein the discs andsleeves may be positioned on the stand by means of threads cut into theexterior surface of the stand and corresponding threads cut into theinterior surface of the sleeves and the interior surface of the holesprovided in the discs.